# What is the wattage?

Computes the wattage of a circuit or appliance from volts and amps (P = V × I, times the power factor for AC), or the amps or volts from the wattage, with kilowatts and volt-amperes.

- Page: https://www.acalculator.org/physics/wattage-calculator
- JSON spec: https://www.acalculator.org/physics/wattage-calculator.json
- Version: 7c7dcd29db51

## Default answer

Example with the default inputs (Find Watts, Current DC, Voltage 120 V, Current 10 A): 120 V at 10 A is 1,200 W.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| find | Find | Which value to work out from the other two. |
| cur | Current | Direct current (batteries, solar, USB) or single-phase alternating current (wall outlets). |
| v | Voltage | The voltage across the load (RMS volts for AC). |
| i | Current | The current through the load (RMS amps for AC). |
| p | Power | The real power the load uses, in watts. |
| pf | Power factor | The share of the volt-amperes that does work: 1 for heaters and bulbs, about 0.8 to 0.95 for motors. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| watts | Wattage | The real power: V × I for DC, V × I × power factor for AC. |
| kilowatts | Kilowatts | The wattage ÷ 1,000. |
| amps | Current | The current in amps. |
| volts | Voltage | The voltage in volts. |
| va | Apparent power | Volts × amps, in volt-amperes; equal to the wattage when the power factor is 1. |

## Method

DC: P = V × I. AC (single phase, RMS): P = V × I × power factor. I = P ÷ (V × pf); V = P ÷ (I × pf); VA = V × I.

## Assumptions

- AC values are RMS (the usual outlet and nameplate values) on a single-phase supply.
- The power factor is 1 for DC and for purely resistive AC loads such as heaters and incandescent bulbs.

## Worked examples

1. find = watts, cur = dc, v = 115, i = 20 gives p = 2,300, kilowatts = 2.3. Source: OpenStax University Physics Volume 2, section 9.5, Electrical Energy and Power, Example 9.9: P = IV = (20.00 A)(115 V) = 2,300 W (https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power, retrieved 2026-10-01).
2. find = watts, cur = ac, v = 120, i = 10, pf = 0.8 gives p = 960, va = 1,200. Source: OpenStax University Physics Volume 2, section 15.4, Power in an AC Circuit: average power = I_rms × V_rms × cos φ, cos φ the power factor (https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit, retrieved 2026-10-01).
3. find = amps, cur = dc, v = 120, p = 1,500 gives i = 12.5.
4. find = volts, cur = ac, i = 5, p = 1,035, pf = 0.9 gives v = 230.

## FAQ

### How do I calculate wattage?

Multiply volts by amps: P = V × I. A motor drawing 20 A at 115 V uses 20 × 115 = 2,300 W. For an AC load that is not purely resistive, also multiply by the power factor.

### How many amps is 1,500 watts?

Divide the watts by the volts: I = P ÷ V. On a 120 V outlet, 1,500 W is 1,500 ÷ 120 = 12.5 A. On 230 V it is about 6.5 A.

### What is a power factor?

In an AC circuit the current and voltage can be out of step. The power factor, cos φ, is the share of volts × amps that does real work. It is 1 for heaters and incandescent bulbs and often 0.8 to 0.95 for motors.

### What is the difference between watts and volt-amperes?

Volt-amperes (VA) are volts × amps, the apparent power. Watts are the real power: VA × power factor. A 120 V, 10 A motor at power factor 0.8 draws 1,200 VA but uses 960 W.

### How do I find the voltage from watts and amps?

Divide the watts by the amps (and by the power factor for AC): V = P ÷ (I × pf). 1,035 W at 5 A and a power factor of 0.9 is 1,035 ÷ 4.5 = 230 V.

### How do I get kilowatt-hours from watts?

Multiply the kilowatts by the hours of use. A 1,500 W heater running 3 hours uses 1.5 × 3 = 4.5 kWh. The electricity cost calculator turns that into money.

## Sources

- OpenStax, University Physics Volume 2, section 9.5, Electrical Energy and Power (P = IV; Example 9.9: 20.00 A at 115 V is 2,300 W), CC BY 4.0, retrieved 2026-10-01. https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power
- OpenStax, University Physics Volume 2, section 15.4, Power in an AC Circuit (average power I_rms V_rms cos φ; cos φ is the power factor), CC BY 4.0, retrieved 2026-10-01. https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit